Yikes! No idle adjustment!

lebolewis · August 6, 2006

Just purchased a 94 GMC suburban with 454 engine with TBI injection. I noticed that it idles a bit too fast. When I stop, I have to hold pressure on the brakes to keep from going forward. If I release the brakes, it will creep forward pretty fast. I have looked everywhere for an idle adjustment and there is NONE! The fast idel burns a lot of GAS!

Any suggestions on what to check/do would be appreciated. Bill

ChevyTech007 · August 7, 2006

I would say you have a vacuum leak. Check your throttle body gaskets. Seems like the older ones have a plate between the throttle body and intake. If yours dose, besure the replace both of them.

lebolewis · August 7, 2006

I would say you have a vacuum leak. Check your throttle body gaskets. Seems like the older ones have a plate between the throttle body and intake. If yours dose, besure the replace both of them.

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OK, I'll take a look to see if there are any leaks. Thanks, Bill

getcrunk88 · August 10, 2006

mabey the iac is sticking open letting too much air into the tb first thing that came to my mind

lebolewis · August 10, 2006

mabey the iac is sticking open letting too much air into the tb first thing that came to my mind

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That would do it, but I checked and that's not the problem. I've tried everything else, checked all the vacuum components, one at a time, removed and plugged, but no help. The only thing left is the TBI gasket or worse yet....the intake manifold gasket (shudder) . Next effort is to replace the TBI gasket. Bill

redvett · August 11, 2006

If you have access to a scanner check the IAC counts should be 15 - 20 if its higher its the TBI gasket.

lebolewis · August 11, 2006

If you have access to a scanner check the IAC counts should be 15 - 20 if its higher its the TBI gasket.

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Don't have access to a scanner. AutoZone has one but only for 96 and newer. I already purchased the new TBI gasket, so I'm going to install it. I haven't figured out how to get the wires disconnected from the two injectors, not familiar with the connector. Maybe I dont have to disconnect? Also the same for the Accelerator cable and another cable connected at the same point. Don't want to break something just getting it disconnected. Can anybody help with these? Also do I have to replace the O-rings in the gas lines or will the old ones be OK? Thanks, Bill

lebolewis · August 11, 2006

If you have access to a scanner check the IAC counts should be 15 - 20 if its higher its the TBI gasket.

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Don't have access to a scanner. AutoZone has one but only for 96 and newer. I already purchased the new TBI gasket, so I'm going to install it. I haven't figured out how to get the wires disconnected from the two injectors, not familiar with the connector. Maybe I dont have to disconnect? Also the same for the Accelerator cable and another cable connected at the same point. Don't want to break something just getting it disconnected. Can anybody help with these? Also do I have to replace the O-rings in the gas lines or will the old ones be OK? Thanks, Bill

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Ok GM Tech, more work done......and the fast idle is still present!!!!!!

I replaced the TBI gasket this morning. It went pretty easy except for the removal of the grommet that feeds the injector wires down through the TB top and out. I couldn't figure how to get that thing out, it's not your ordinary grommet, it's odd shaped and it's special made. I didn't want to damage it.....probably cannot get a replacement easily. I finally got it out with minimal damage. The 2 air filter housing mounting rods do indeed screw into the top of 2 of the 3 TBI mounting bolts, on my version. I temporarily removed them so I was able to torque to spec all three TB mounting bolts. Reassembled everything and still have idle at 1000 RPMs in neutral and 800 RPMs in gear. I suspected as much when I examined the old TB gasket and found it to be in very good shape. But I did find a HOLE where you can see the pintle of the IAC and I noticed that the IAC was open in it's "at rest" position. With everything reassembled, and the engine running, I sprayed WD40 into the Hole and the engine speeds up, momentarily. That tells me that the IAC is "open", even with the engine running at 1000 RPM idle!! Does that suggest a defective IAC?? .....or what??

redvett · August 12, 2006

The gasket between the TBI & manifold is probably the badone but the but the IAC can be bad too and it should be closed if you shut off the running engine and remove the TBI and look at the seat. It should be closed. If not its bad.

GM-Tech · August 12, 2006

If you have access to a scanner check the IAC counts should be 15 - 20 if its higher its the TBI gasket.

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Actually, IAC counts go down, not up, with a vacuum leak. An IAC count of 0 is an excellent indicator of a major vacuum leak. It reads 0 because the computer is attempting to slow the engine down by shutting off IAC air (0 counts = 100% closed).

Also the IAC will not be closed (assuming it's working normally) if you shut the engine off, take it apart and look. The computer retracts (opens) the IAC on shut down in preparation for the next start.

To know for sure, unplug the IAC while the vehicle is idling, then shut it off. The computer can't do anything with it disconnected and you can see what state it's in at idle... which may or may not tell you anything.

Blown TBI gaskes are common on these... that's the first place I'd look.

lebolewis · August 12, 2006

If you have access to a scanner check the IAC counts should be 15 - 20 if its higher its the TBI gasket.

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Actually, IAC counts go down, not up, with a vacuum leak. An IAC count of 0 is an excellent indicator of a major vacuum leak. It reads 0 because the computer is attempting to slow the engine down by shutting off IAC air (0 counts = 100% closed).

Also the IAC will not be closed (assuming it's working normally) if you shut the engine off, take it apart and look. The computer retracts (opens) the IAC on shut down in preparation for the next start.

To know for sure, unplug the IAC while the vehicle is idling, then shut it off. The computer can't do anything with it disconnected and you can see what state it's in at idle... which may or may not tell you anything.

Blown TBI gaskes are common on these... that's the first place I'd look.

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I have already replaced the TBI gasket (see above). And, I have confirmed that the IAC is open at idle, even with the engine idleing at 1000 RPMs. And, I have checked the Coolant Sensor and it's getting 5.2 volts from the Computer and measures 980 ohms when the engine is warm to the touch. The Haynes Manual says that is about normal. I will look again this morning to see it there is a SECOND gasket below the TBI under some kind of adapter.....but I don't think there is. Any ideas? Bill

lebolewis · August 12, 2006

I have already replaced the TBI gasket (see above). And, I have confirmed that the IAC is open at idle, even with the engine idleing at 1000 RPMs. And, I have checked the Coolant Sensor and it's getting 5.2 volts from the Computer and measures 980 ohms when the engine is warm to the touch. The Haynes Manual says that is about normal. I will look again this morning to see it there is a SECOND gasket below the TBI under some kind of adapter.....but I don't think there is. Any ideas? Bill

GM-Tech · August 12, 2006

Can you get a scanner on this thing and see what the IAC counts are when it's idling abnormally high? If something external (vacuum leak, etc) is causing it, the ECM should be attempting to drive the IAC fully seated (0 counts) to lower the idle.

If the IAC counts are in a more normal range, say 15-30 (very rough), then the ECM believes the idle speed is just fine and is not trying any heroic measures with the IAC to lower it down.

lebolewis · August 12, 2006

Can you get a scanner on this thing and see what the IAC counts are when it's idling abnormally high? If something external (vacuum leak, etc) is causing it, the ECM should be attempting to drive the IAC fully seated (0 counts) to lower the idle.

If the IAC counts are in a more normal range, say 15-30 (very rough), then the ECM believes the idle speed is just fine and is not trying any heroic measures with the IAC to lower it down.

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Thanks for responding. I think it is like you stated in your last paragraph....."the ECM believes the idle speed is just fine and is not trying any heroic measures with the IAC to lower it down." Here's why: This morning, I was going to do the test you mentioned for the IAC position "engine off" closed or open? So, with engine on and at 1000 RPMs idle, and using a mirror and light, I looked down the air port, that is just above the IAC on top of the TB, to see if I could tell what position the IAC pentle was in. I could just barely see the tip of the pintle, indicating a partially open position. I still had the PCV vacuum port plugged with a golf tee, (the results of other tests), so I removed the tee and the idle speed increased considerably and then settled back at about 1200 RPMs. I re-checked the IAC pentle position and it was fully closed, purtruding noticably into the air port. The computer was trying to get the idle back down to 1000 RPMs by commanding the IAC to close. That suggests that the IAC is following the commands of the computer and is able to move back and forth, including the fully closed position. Something else is telling the computer that the idle speed is OK at 1000 RPMs, when it is NOT!! Question still is, what is doing that? Any more ideas? I've already checked the Coolant Sensor and it checks OK. Bill